This invention relates to a device and method for improving heat transfer between a web and a chill drum in a vacuum chamber.
Many vacuum deposition processes involving flexible web substrates are accomplished with the web disposed around a rotating chilled drum. In these systems, deposition sources are arrayed around the drum and continuously deposit coatings onto the moving web. A limiting parameter of these deposition processes is the heat imparted to the web during the coating process. If the heat applied by the process exceeds maximum web parameters, the web wrinkles or is otherwise damaged. Many products today are either expensive or are not produced because of low deposition rates dictated by insufficient web heat transfer.
Heat removal from a web to a chilled drum is primarily limited by the interface between the web and drum. In this interface, heat is transferred by three modes. One mode is conduction between the two surfaces. Typical polymer webs are not smooth at the micron level. They are made intentionally rough to allow the film to be wound on a spool. While improving ease of handling, this surface roughness greatly reduces the actual contact between the web and the drum. Lack of contact in turn limits the heat transfer by conduction to less than 5% of the total heat transfer. A second heat transfer mode is radiation. While also contributing to heat removal, heat removed by this mode is limited by the relatively small temperature difference possible between the web and drum.
The third and largest contributor to heat transfer between the web and drum is molecular conduction. This mode occurs when molecules trapped between the web and drum transfer heat between the two surfaces. Commonly water vapor is present in polymer substrate films and devolves from the substrate during the deposition process. A portion of this water vapor is trapped between the web and drum and provides a medium for molecular conduction heat transfer. Important factors determining the rate of molecular conduction heat transfer include: the temperature of the web and drum, the web and drum materials, the type of gas, and the pressure of the gas. Variations to web and drum temperatures and materials are limited by materials properties; however, a significant opportunity for improvements in heat transfer resides with the type of gas and the pressure of the gas. If these variables can be optimized, the heat load into the web can be increased without damage to the web. Because the pressure can be varied by orders of magnitude, it offers the best lever for dramatic heat transfer improvement.
Several prior-art devices have attempted to improve the web-to-drum heat transfer by elevating the pressure between the web and drum:
U.S. Pat. No. 3,414,048 (Rall) discloses a drum with built in normally closed valves. Web in contact with the drum forces open the valves, allowing gas to flow into the gap between the web and drum. This apparatus is complicated with many parts to stick or fail. Also a thin polymer web may fail to exert sufficient pressure on the valve to open them. Other limitations of this approach include hot spots on the web (the valves are not cooled) and non-uniformity of web cooling.
U.S. Pat. No. 5,076,203 (Vaidya) discloses apparatus to increase the pressure behind the web by blowing gas into the gap with a nozzle arrangement. Another method to increase pressure employs a porous metal non-rotating section through which gas is distributed. An enclosure around the web and drum at the entrance point of the web is shown as a means to limit the increase in chamber pressure as gas is urged into the gap. While informative, several faults limit the utility of this device and method:                No means is described to continually trap working gas behind the web in a rotating drum configuration. Due to the minute quantity of gas trapped as the drum rotates away from the nozzle area, most or all trapped gas is lost before reaching the rewind side of the chamber. Therefore, the enclosure that is used to prevent gas from raising the system pressure only envelopes the unwind drum side. This indicates the small amount of gas actually urged into the web. High pressures would result in large gas loads on the chamber vacuum system.        No means is disclosed to bring a working gas into a porous material where the porous material is applied to the surface of a rotating drum.        No means is disclosed to alter the pressure effectively across the width of the web for the purpose of controlling web heat transfer and conveyance parameters. Distribution of gas in the porous surfaces is across the width of the drum.        
In U.S. Pat. No. 5,395,647 (Krug), a vapor such as water is condensed onto the web prior to contact with the chill drum. While recognizing the need to improve heat transfer between the web and drum, this method lacks practicality for most deposition processes. The use of liquid water creates an undesirably large gas load on the pumping system, and uniformly dispensing of water vapor in vacuum is difficult. While clean and of a suitable vapor pressure, water vapor is detrimental to the formation of many desirable films. If a low vapor pressure fluid other than water is used, the web becomes contaminated with the substance.